There are, in general, two ways of flowing air into a combustion zone in a stove or the like.
One of these ways can be called the "up-draft method". In the up-draft method, the air to support combustion is introduced upwardly through a grate on which the combustible material is located which is to be consumed to produce heat. The products of combustion, in the up-draft method, flow from the top of the combustible material upwardly through the stove or other apparatus and into a chimney. This method is relatively inefficient because smoke, which contains much conbustible carbon, and fumes or vapors, which also contain much combustible hydrocarbon material, largely escape unburned from the firebed.
The second general method may be called the "down-draft method". In the down-draft method, air is introduced into the stove or other combustion apparatus above the firebed and flows downwardly through the firebed and the grate to a plenum chamber or other exhaust system to a chimney. In the down-draft method, the products of combustion from the fresher fuel on top of the bed of burning embers passes through the firebed where the combustible part thereof, including smoke and vapors, has an opportunity to burn. The down-draft method is, therefore, generally regarded by those skilled in the art as the more efficient of the two general methods.
Combustion apparatuses are shown in the prior art capable of carrying out each of these methods and combinations of them.
The following patents are referred to by way of example of such prior art:
Ayres U.S. Pat. No. 839,804, patented Jan. 1, 1907, discloses a stove with a grate located approximately one-third of the distance from the bottom to the top of the stove. The space below the grate forms a plenum chamber having a connection to a stove pipe or chimney. The space above the grate also has a connection from the upper portion thereof to the same stovepipe or chimney. There is a damper on the top of the stove leading to an air inlet tube which terminates about halfway between the top of the stove and the grate so that air entering through the damper in the top of the stove is funneled to a sort of jet that impinges upon the fuel resting upon the grate. There is a fuel inlet opening in the top of the stove covered by a hinged door and an ash outlet opening near the bottom of the stove closed by a hinged door which also can serve as an air inlet by means of a valve or damper arrangement in the door. The stove may operate as an updraft stove by closing a damper in the lower connection of the chimney, opening the damper in the ash door, closing the damper in the top of the stove, and opening the damper in the upper connection to the chimney. The stove can also operate as a down draft combustion apparatus by reversing the positions of the four dampers.
Jordan U.S. Pat. No. 973,201, patented Oct. 18, 1910, discloses a heating stove having some similarity to the Ayres stove except that it does not have a damper in the top wall of the stove nor an air inlet tube. On the contrary, the front of the stove is provided with a fuel door in the upper part thereof with a damper in it and two doors near the bottom with dampers in them, the upper door of the two giving access to an ash pit and the lower of them giving access to an air-box. There is a lower outlet from a plenum below the grate comprised of the ash pit and the air-box, and an upper outlet from the space above the grate leading to a common stove pipe which is connected with a chimney or the like. This heating stove can also operate either on the up-draft or down-draft principle by suitable manipulation of the dampers in each outlet and in the three doors mentioned.
Atterberry U.S. Pat. No. 1,044,724, patented Nov. 19, 1912, discloses a stove which operates only on the down-draft principle. The stove comprises a top having air inlet dampers, a fuel inlet opening provided with cover over it, a grate, a plenum chamber and ash pit below the grate with an opening leading into the ash pit which is normally closed by a door having a damper in it. Air is supplied to the space above the grate not only through the dampers in the top of the stove but also through a special conduit which collects air near the floor of the room in which the stove is located, or from any other convenient place, and introduces it high above the bed of fuel burning in the space above the grate.
Brooks U.S. Pat. No. 1,987,548, issued Jan. 8, 1935, discloses a heater which also can work on the up-draft or down-draft principle. The preferred operation is by the down-draft principle with the products of combustion flowing downwardly through the grate and into a manifold or plenum chamber at the bottom of the stove which is connected to a plurality of vertical tubes located at the sides and back of the stove. These tubes, which communicate at their upper ends with a plenum chamber connected to a chimney collar, are spaced from the stove body far enough to permit air circulation completely around them and thereby increase the heat transfer surface for the heat conducted from the products of combustion through the walls of the tubes to the surrounding atmosphere.
Kapustion U.S. Pat. No. 3,874,362, issued Apr. 1, 1975, discloses a space heating device for burning solid fuel which operates on the up-draft principle but has alternate outlets for products of combustion.
Runquist U.S. Pat. No. 4,102,318, discloses a combustion furnace which operates on the down-draft principle with a special reactor chamber below the grate which is intended to assure complete combustion of the products of combustion. This includes a small opening in each sidewall of the furnace at the level of the reactor chamber below the grate so that a small amount of fresh air is introduced laterally into the reactor chamber where only gases are present. The top of the furnace has a hinged airtight lid for introduction of the solid fuel into the chamber above the grate which is called the primary combustion chamber. The front of the furnace has a hinged door also opening into the primary combustion chamber which can be opened for starting the fire. It is provided with a dampered opening for the introduction of air from the room into the primary combustion chamber through which it flows in downward direction through the grate and the reactor chamber and then out a heat exchange chamber near the back of the stove and into the chimney. Back of the heat exchange chamber is an air heating chamber into which air enters at the bottom from the room or outside of the room, flows upwardly through the heating chamber and out into the room. This air is not exposed at any time to products of combustion but only to the heated wall of the heat exchange chamber behind the firebox and in front of the heating chamber for the room air.
All of these combustion devices of the prior art suffer from a number of disadvantages. One is that they require continual attention to adjust dampers for controlling the heat output and for supplying fuel in relatively limited quantities to the combustion chamber or firebox. Loading these stoves with a relatively large quantity of fuel to burn over a much longer period of time is not considered practical because after loading the fuel into the combustion apparatus it either (a) soon all ignites and burns, producing more heat than necessary or (b) snuffs out the fire as is most often the case with small particulate fuel because it blocks off the passage of enough air to support combustion, either by updraft or downdraft operation, and thus burns quite inefficiently with large output of smoke and combustible vapors which escape unburned into the chimney.